The high frequency clock signals used in many electronic circuits generate coincidental electromagnetic fields. These coincidental electromagnetic fields or emissions are known as electromagnetic interference or “EMI. ”The energy of EMI emissions from a particular circuit is directly proportional to the frequency of the clock signal used in the circuit.
EMI emissions from electronic devices are regulated by governmental or other agencies to certain maximum allowable limits. Electronic equipment manufacturers must reduce EMI emissions as much as possible in order to maintain these emissions within the allowable limits. For example, electronic circuits may be enclosed in special electrically conductive housings which block or shield EMI emissions from the enclosed circuit. However, as clock frequencies increase, complete EMI shielding becomes more difficult, and EMI emission levels may increase.
Many regulatory EMI limits are set as a maximum average emission energy level over an operating period for the circuit. Reducing clock frequency in a circuit for a portion of an operating period reduces the average energy of EMI emissions over the operating period. Thus, it is sometimes possible to meet regulatory limits for EMI emissions from a particular circuit by modulating the clock frequency in the circuit within a certain range about a centerline or nominal clock frequency. This modulated clock signal frequency in an electronic circuit is commonly referred to as a spread spectrum clock signal.
All processors and other electronic devices that operate under control of a clock signal are limited in the clock frequency they can support, and thus the speed at which they can process data. This operational speed limit is the result of certain critical paths between functional blocks which are clocked by a common clock signal. The maximum time it takes to launch data from one circuit functional block, transmit it to a receiving circuit functional block, and arrive at the receiving circuit functional block prior to the setup time required by the receiving circuit functional block, determines the minimum instantaneous cycle time that may be allowed for a clock signal in an electronic device which includes the two functional blocks. This minimum instantaneous cycle time translates to a maximum clock frequency for the circuit and can never be violated without running the risk that the circuit will produce incorrect results for a given input. Where the clock frequency for a circuit is modulated in a spread spectrum clock arrangement, the modulated frequency must be controlled so that the instantaneous frequency at any given time remains below the maximum allowable clock frequency supported by the circuit.
Minimum cycle time may be improved or reduced in many circuits by increasing the supply voltage in the circuit. Thus, the maximum allowable instantaneous frequency in a spread spectrum clock signal may be increased simply by increasing the supply voltage to the circuit. However, increasing the supply voltage in a circuit will increase power dissipation in the circuit, and the thermal effect of this increased power dissipation may have a detrimental impact on the functionality and reliability of the circuit. Thus, in many systems, increasing supply voltage level is not a viable choice to meet limitations on the maximum instantaneous frequency in a spread spectrum clock system.